As is known, LED sources are increasingly widespread, since they are characterized, among other things, by a high energy efficiency and a low power consumption given the same brightness.
LED sources require driving circuits capable of supplying d.c. currents at a low voltage. For this reason, in the case where a LED source is to be supplied through the electric power grid, it is necessary to use, within the driving circuit, a switching converter, such as, for example, a converter of a buck, boost, or flyback type.
Use of switching converters is particularly indicated in the case of professional applications, i.e., in the case of applications in which the level of power required is relatively high (for example, higher than 50 W), and in which the constraints regarding the packaging and installation are not stringent. Instead, in the case of applications (for example) in the domestic field, the powers required are low, and integration in switching-converter driving circuits is problematic, since the constructional constraints for LED sources, for example as regards the corresponding plugs, are stringent.
As an alternative to the use of switching converters, less complex solutions have been proposed, also known as AC-LEDs. These solutions present some aspects in common, such as: the presence of a rectifier circuit; the presence of a plurality of LED strings, each string being formed by a corresponding number of LEDs connected in series; and the presence of one or more modules, which regulate the current that flows in the strings as a function of the value of the sinusoidal grid voltage. An example of driving circuit of an AC-LED type is described in European Patent No. 2645816 (incorporated by reference).
In greater detail, typically a driving circuit of an AC-LED type is configured in such a way that, as the sinusoidal grid voltage increases, the number of LED strings connected in series increases, and consequently the number of LEDs that are on. Further, as the number of on LEDs increases, the driving circuit increases the regulated current. More in particular, the current increments occur according to discrete levels; the current thus remains constant for a certain time interval, before rising to the next level. The number of current levels is equal to the number of LED strings.
This being said, driving circuits of the so-called AC-LED type are effectively characterized by a high constructional simplicity. However, they provide only discrete levels of performance as regards flicker of the visible radiation generated thereby, on account of the stepwise plot of the current. In this connection, traditionally light flicker is expressed via two quantities: flicker percent and flicker index. In particular, given one period of the supply voltage, the flicker percent is equal to 100·(A−B)/(A+B), where A and B are the maximum and minimum values of luminous flux, during the period. Instead, the flicker index is equal to AREA1/(AREA1+AREA2), where, given an area subtended by the curve that represents the luminous flux during a period of the supply voltage, AREA1 is the portion of area that exceeds the mean value of the luminous flux in said period, whereas AREA2 is the portion of area that is lower than said mean value. In the case of AC-LED systems, typically it is possible to obtain values of flicker index and flicker percent of, for example, 0.34 and 99%, respectively.
Since light flicker may be harmful for human health and further interferes with filming and photographing, it is desirable for it to be as low as possible. In order to reduce the flicker index, the U.S. Pat. No. 8,742,682 (incorporated by reference) suggests coupling the LED strings to capacitors with high values of capacitance (of the order of hundreds of microfarads). In this way, however, there occurs a reduction of the service life of the lamp, on account of the reduced service life of said capacitors, as well as a deterioration of other characteristic parameters of the AC-LED system, such as efficiency. In addition, the circuit proposed presents problems of integration.
There is a need in the art to provide an electronic driving circuit that will solve at least in part the foregoing drawbacks.